All batteries require a battery separator which is used to separate positive and negative electrode at the same time allowing ionic conduction. The separator base web or back web thickness and overall thickness which is thickness of the separator with rib or glass mat or both, depends on type of separator, type of application and life of the battery expected.
Most polyethylene separators are produced with a variety of rib designs, which are provided on either one or both sides of the back web. The thickness, width and angle of rib vary from application to application and from brand to brand. Most automotive separators carry a rib of 0.7 mm and they are placed generally 1 cm apart and they are present all along the length of the separator. These ribs help creating extra space between the electrodes to provide additional volume of electrolyte which is required for the performance of the battery. In applications such as stationary and traction applications, the rib thickness can be as high as 1.2 mm and in number of designs they are present on both sides of the back web. This is done considering the life expectancy of the battery, requirement of higher amount of electrolyte between the electrodes.
Typically the ribs are provided on polyethylene separators by an on line process wherein the extruded polyethylene separator web is passed between a pair of calendared rolls carrying grooves which produce ribs on the polyethylene when it passes through the calendar. For every change in design and dimension of the rib, a new set of specially designed pair of calendared rolls have to be used.
Similarly, the PVC separators are provided with ribs on either one or both sides depending on end use of the battery. Typical thickness of such ribs is around 1.0 mm and in some applications the final thickness of the separator can be as high as 4 mm. In sintered PVC separators, the ribs are provided by doctoring the sintered resin through a grooved roll. The grooves actually create a continuous rib on the separator. In case of extruded or calendared PVC separators, the process of forming rib is similar to polyethylene where the pair of calendar rolls carries the impression of the desired rib design. There are also separators which are rib less but use corrugation as a process to increase the overall thickness of the separator. In such cases also, the total weight of separator increases due to increase area of separator consumed in the process of corrugation.
Hence, the weight of rib in separators is considerable. Cost of making a new rib is also expensive as it demands change of new calendar rolls and the electrolyte displaced due to the additional volumes of these rib materials is also higher. It is well known in battery design that the separators should have lowest acid displacement. Higher the acid displaced, lower is the battery performance.
Most type of separators made by wet laid technology such as SPG separators resin impregnated cellulose and other composite separators heavily rely on use of glass mat, very thick back web, rarely on extruded rib or combination of these. Most of these separators require a glass mat or a rib or both depending on end application and also due to the limitation of the composition such as degradation of oxidation, higher pore size etc.
Most wet laid separators vary their thickness by making changes to the thickness of the back web and thickness of the glass mat. As the thickness of the back web or glass mat increases, the cost of the separator increases substantially. And in applications where both back web, glass mat thickness and glass mat and rib is all used in combination, the cost of the separator increases substantially. The thickness increase achieved by these techniques not only increase the cost but also reduce the performance of the battery as the acid displaced is high and more mass of material in between electrodes means higher electrical resistance of the separator which in turn effects the performance of the battery.
It was observed from the prior art search that there are no separators or separator making technologies other than use of a rib or glass mat to increase the overall thickness of the separator. It is possible that all existing techniques of separator production are limited either to use a continuous rib or a glass mat or both to increase the overall thickness of the separator. The purpose of a glass mat or rib is only to make provision for additional electrolyte but at a considerable cost addition. Any attempt to reduce length of a rib is limited by the hitherto known production techniques or alternative methods not thought off.